The function of skeletal alkaline phosphatase (ALP) in vivo is unknown. There are two reasons why the enzyme is thought to play a role in bone formation: (1) it is localized in the membranes of osteoblasts, and (2) its activity in serum seems to reflect the bone formation rate. Corresponding to these reasons, there are two objectives to the proposed studies: (1) to determine the function and the regulation of skeletal ALP in bone (i.e.; in osteoblasts), and (2) to determine the usefulness of measuring skeletal ALP in serum as an index of skeletal metabolism. To accomplish the first goal I will combine two complimentary approaches: (a) I will critically evaluate the several proposed functions of skeletal ALP that are consistent with our current understanding, and (b) I will investigate the regulation of skeletal ALP by various effectors of mineral metabolism (i.e; NaF, PTH, vitamin D, etc.), comparing the effects of those agents on ALP activity with their effects on bone growth and bone cell proliferation in vitro. These studies will involve both tissue culture and organ culture (i.e; bone cells and bone). Cell proliferation will be measured by the incorportion of 3H-thymidine into DNA and bone growth will be measured by collagen production and by the incorporation of 45Ca into bone mineral. To accomplish the second goal I will develop a selective immunoassay for skeletal ALP in serum to compliment the similarly selective kinetic assay I have already developed. (Skeletal ALP will be purified from human bone and antibodies will be prepared in rabbits and in mice; monoclonal antibodies will be derived from the mice.) I will use these two assays to evaluate the correlations between skeletal ALP in serum and various parameters of bone growth; particularly osteoblast number and activity. These studies will involve animal, human and in vitro investigations. The two objectives will be integrated by studies into the mechanism(s) by which skeletal ALP is released from osteoblasts into circulation. It is also anticipated that these studies will define the relationships between skeletal ALP in bone and skeletal ALP in bone and skeletal ALP in serum so that the serum activity may be used as an index of skeletal metabolism for the assessment and management of metabolic bone disease.